1 Lecture 11 protection and Security Operating System Fall 2005

2 Contents Protection Goals of Protection Principles of Protection Domain of Protection Access Matrix Implementation of Access Matrix Access Control Revocation of Access Rights Security The Security Problem Program Threats Cryptography as a Security Tool User Authentication

3 Goals of Protection Operating system consists of a collection of objects, hardware or software Each object has a unique name and can be accessed through a well-defined set of operations. Protection problem - ensure that each object is accessed correctly and only by those processes that are allowed to do so.

4 Principles of Protection Guiding principle – principle of least privilege Programs, users and systems should be given just enough privileges to perform their tasks

5 Domain Structure Access-right = where rights-set is a subset of all valid operations that can be performed on the object. Domain = set of access-rights

6 Access Matrix View protection as a matrix (access matrix) Rows represent domains Columns represent objects Access(i, j) is the set of operations that a process executing in Domain i can invoke on Object j

7 Access Matrix

8 Access Matrix of Figure A With Domains as Objects Figure B

9 Access Matrix with Copy Rights

10 Access Matrix With Owner Rights

11 Modified Access Matrix of Figure B

12 Use of Access Matrix If a process in Domain D i tries to do “op” on object O j, then “op” must be in the access matrix. Can be expanded to dynamic protection. Operations to add, delete access rights. Special access rights: owner of O i copy op from O i to O j control – D i can modify D j access rights transfer – switch from domain D i to D j

13 Implementation of Access Matrix Each column = Access-control list for one object Defines who can perform what operation. Domain 1 = Read, Write Domain 2 = Read Domain 3 = Read  Each Row = Capability List (like a key) Fore each domain, what operations allowed on what objects. Object 1 – Read Object 4 – Read, Write, Execute Object 5 – Read, Write, Delete, Copy

14 Access Control Protection can be applied to non-file resources Solaris 10 provides role-based access control to implement least privilege Privilege is right to execute system call or use an option within a system call Can be assigned to processes Users assigned roles granting access to privileges and programs

15 Role-based Access Control in Solaris 10

16 Revocation of Access Rights Access List – Delete access rights from access list. Simple Immediate Capability List – Scheme required to locate capability in the system before capability can be revoked. Reacquisition Back-pointers Indirection Keys

17 The Security Problem Security must consider external environment of the system, and protect the system resources Intruders (crackers) attempt to breach security Threat is potential security violation Attack is attempt to breach security Attack can be accidental or malicious Easier to protect against accidental than malicious misuse

18 Standard Security Attacks

19 Program Threats: C Program with Buffer-overflow Condition #include #define BUFFER SIZE 256 int main(int argc, char *argv[]) { char buffer[BUFFER SIZE]; if (argc < 2) return -1; else { strcpy(buffer,argv[1]); return 0; }

20 Layout of Typical Stack Frame

21 Program Threats: viruses Code fragment embedded in legitimate program Very specific to CPU architecture, operating system, applications Usually borne via or as a macro Visual Basic Macro to reformat hard drive Sub AutoOpen() Dim oFS Set oFS = CreateObject(’’Scripting.FileSystemObject’’) vs = Shell(’’c:command.com /k format c:’’,vbHide) End Sub

22 Program Threats (Cont.) Virus dropper inserts virus onto the system Many categories of viruses, literally many thousands of viruses File Boot Macro Source code Polymorphic Encrypted Stealth Tunneling Multipartite Armored

23 Cryptography as a Security Tool Broadest security tool available Source and destination of messages cannot be trusted without cryptography Means to constrain potential senders (sources) and / or receivers (destinations) of messages Based on secrets (keys)

24 Secure Communication over Insecure Medium

25 Encryption Encryption algorithm consists of Set of K keys Set of M Messages Set of C ciphertexts (encrypted messages) A function E : K → (M → C). That is, for each k  K, E(k) is a function for generating ciphertexts from messages. Both E and E(k) for any k should be efficiently computable functions. A function D : K → (C → M). That is, for each k  K, D(k) is a function for generating messages from ciphertexts. Both D and D(k) for any k should be efficiently computable functions. An encryption algorithm must provide this essential property: Given a ciphertext c  C, a computer can compute m such that E(k)(m) = c only if it possesses D(k). Thus, a computer holding D(k) can decrypt ciphertexts to the plaintexts used to produce them, but a computer not holding D(k) cannot decrypt ciphertexts. Since ciphertexts are generally exposed (for example, sent on the network), it is important that it be infeasible to derive D(k) from the ciphertexts

26 Symmetric Encryption Same key used to encrypt and decrypt E(k) can be derived from D(k), and vice versa DES is most commonly used symmetric block-encryption algorithm (created by US Govt) Encrypts a block of data at a time Triple-DES considered more secure Advanced Encryption Standard (AES), twofish up and coming RC4 is most common symmetric stream cipher, but known to have vulnerabilities Encrypts/decrypts a stream of bytes (i.e wireless transmission) Key is an input to psuedo-random-bit generator Generates an infinite keystream

27 Asymmetric Encryption Public-key encryption based on each user having two keys: public key – published key used to encrypt data private key – key known only to individual user used to decrypt data Must be an encryption scheme that can be made public without making it easy to figure out the decryption scheme Most common is RSA block cipher Efficient algorithm for testing whether or not a number is prime No efficient algorithm is know for finding the prime factors of a number

28 Asymmetric Encryption (Cont.) Formally, it is computationally infeasible to derive D(k d, N) from E(k e, N), and so E(k e, N) need not be kept secret and can be widely disseminated E(k e, N) (or just k e ) is the public key D(k d, N) (or just k d ) is the private key N is the product of two large, randomly chosen prime numbers p and q (for example, p and q are 512 bits each) Encryption algorithm is E(k e, N)(m) = m k e mod N, where k e satisfies k e k d mod (p−1)(q −1) = 1 The decryption algorithm is then D(k d, N)(c) = c k d mod N

29 Asymmetric Encryption Example For example. make p = 7and q = 13 We then calculate N = 7 ∗ 13 = 91 and (p−1)(q−1) = 72 We next select k e relatively prime to 72 and< 72, yielding 5 Finally,we calculate k d such that k e k d mod 72 = 1, yielding 29 We how have our keys Public key, k e, N = 5, 91 Private key, k d, N = 29, 91 Encrypting the message 69 with the public key results in the cyphertext 62 Cyphertext can be decoded with the private key Public key can be distributed in cleartext to anyone who wants to communicate with holder of public key

30 Encryption and Decryption using RSA Asymmetric Cryptography

31 Cryptography (Cont.) Note symmetric cryptography based on transformations, asymmetric based on mathematical functions Asymmetric much more compute intensive Typically not used for bulk data encryption

32 Authentication Constraining set of potential senders of a message Complementary and sometimes redundant to encryption Also can prove message unmodified Algorithm components A set K of keys A set M of messages A set A of authenticators A function S : K → (M → A) That is, for each k  K, S(k) is a function for generating authenticators from messages Both S and S(k) for any k should be efficiently computable functions A function V : K → (M× A → {true, false}). That is, for each k  K, V(k) is a function for verifying authenticators on messages Both V and V(k) for any k should be efficiently computable functions

33 Authentication (Cont.) For a message m, a computer can generate an authenticator a  A such that V(k)(m, a) = true only if it possesses S(k) Thus, computer holding S(k) can generate authenticators on messages so that any other computer possessing V(k) can verify them Computer not holding S(k) cannot generate authenticators on messages that can be verified using V(k) Since authenticators are generally exposed (for example, they are sent on the network with the messages themselves), it must not be feasible to derive S(k) from the authenticators

34 Authentication – Hash Functions Basis of authentication Creates small, fixed-size block of data (message digest, hash value) from m Hash Function H must be collision resistant on m Must be infeasible to find an m’ ≠ m such that H(m) = H(m’) If H(m) = H(m’), then m = m’ The message has not been modified Common message-digest functions include MD5, which produces a 128-bit hash, and SHA-1, which outputs a 160-bit hash

35 Authentication - MAC Symmetric encryption used in message- authentication code (MAC) authentication algorithm Simple example: MAC defines S(k)(m) = f (k, H(m)) Where f is a function that is one-way on its first argument k cannot be derived from f (k, H(m)) Because of the collision resistance in the hash function, reasonably assured no other message could create the same MAC A suitable verification algorithm is V(k)(m, a) ≡ ( f (k,m) = a) Note that k is needed to compute both S(k) and V(k), so anyone able to compute one can compute the other

36 Authentication – Digital Signature Based on asymmetric keys and digital signature algorithm Authenticators produced are digital signatures In a digital-signature algorithm, computationally infeasible to derive S(k s ) from V(k v ) V is a one-way function Thus, k v is the public key and k s is the private key Consider the RSA digital-signature algorithm Similar to the RSA encryption algorithm, but the key use is reversed Digital signature of message S(k s )(m) = H(m) k s mod N The key k s again is a pair d, N, where N is the product of two large, randomly chosen prime numbers p and q Verification algorithm is V(k v )(m, a) ≡ (a k v mod N = H(m)) Where k v satisfies k v k s mod (p − 1)(q − 1) = 1

37 Authentication (Cont.) Why authentication if a subset of encryption? Fewer computations (except for RSA digital signatures) Authenticator usually shorter than message Sometimes want authentication but not confidentiality Signed patches et al Can be basis for non-repudiation

38 Key Distribution Delivery of symmetric key is huge challenge Sometimes done out-of-band Asymmetric keys can proliferate – stored on key ring Even asymmetric key distribution needs care – man-in-the-middle attack

39 Man-in-the-middle Attack on Asymmetric Cryptography

40 Digital Certificates Proof of who or what owns a public key Public key digitally signed a trusted party Trusted party receives proof of identification from entity and certifies that public key belongs to entity Certificate authority are trusted party – their public keys included with web browser distributions They vouch for other authorities via digitally signing their keys, and so on

41 End of lecture 11 Thank you!